Pressure vessel systems are in widespread usage in a variety of applications, including heavy duty truck brake systems. In general, fluid or gas is maintained under pressure in a welded vessel, and a pressure pipe coupled to the vessel distributes the pressurized fluid or gas to an end use device, such as a brake cylinder. The pipe may be coupled to the pressure vessel in various ways, including threaded fittings, and sealed plug-in fittings. While threaded fittings are easily removed for system maintenance and repair, they also tend to leak, resulting in system down-time for repairs. Plug-in fittings, on the other hand, use one or more O-rings to seal against a smooth bore of a receiver or ferrule that is attached to the pressure vessel, and are typically retained in place with a snap ring which radially expands into a ring groove formed in the bore of the ferrule. Compared to threaded fittings, plug-in fittings provide a significantly more reliable coupling, in addition to being capable of quick installation. Unfortunately, however, plug-in fittings tend to be relatively expensive due to the required machining of the ferrule, and are not generally amenable to welding due to heat-related distortion of the ferrule bore which forms the sealing surface for the O-rings. Accordingly, what is needed is a plug-in fitting that provides the reliable sealing advantages of known plug-in fittings, but which is inexpensive to manufacture and amenable to welding.